Synthesis and Characterization of a Stable trans-Dioxo Tungsten(IV) Complex and Series of Mono-Oxo Molybdenum(IV) and Tungsten(IV) Complexes. Structural and Electronic Effects of pi-Bonding in trans-[M(O)(X)(dppe)(2)](+/0) Systems.

trans-[W(O)(F)(dppe)(2)](BF(4)) (dppe = Ph(2)PCH(2)CH(2)PPh(2), 1,2-bis(diphenylphosphino)ethane) (10) has been synthesized. From this, the neutral trans-[W(O)(2)(dppe)(2)].2CH(3)OH (11) was prepared by a hydrolysis and deprotonation reaction with Et(4)NOH in methanol/water. Together with the analogous molybdenum compound (2) this compound afforded by substitution under acid conditions salts of trans-[M(O)(X)(dppe)(2)](+) ions (M = Mo, W; X = OH, OCH(3), Cl, Br, I, NCS) and trans-[Mo(O)(N(3))(dppe)(2)](CF(3)SO(3)) (9). All the compounds have low-spin d(2) electronic configuration. The compounds were characterized by (31)P{(1)H} NMR, positive FAB-ionization mass spectrometry, UV-vis spectroscopy, and vibrational spectroscopy. X-ray crystallographic studies were carried out on compounds 10, 11, trans-[W(O)(OH)(dppe)(2)](ClO(4)) (12), and trans-[W(O)(NCS)(dppe)(2)](BPh(4)) (16). The electronic spectra of the mono-oxo species are consistent with the lowest energy transitions being of d(xy)() --> {d(zx)(), d(yz)()} character. No ligand-field transitions are observed for the dioxo tungsten complex. Identical pi-spectrochemical series were found for the two metals. The shielding of the phosphorus nuclei determined by (31)P{(1)H} NMR is significantly influenced by the nature of the axial ligands.